The Semiconductor DiodeMay
1961 Popular Electronics

Wax nostalgic about and learn from the history of early electronics. See articles
from Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.

"What are these devices?" "How do they work?" "What are their
characteristics?" "How are they used?" Those are the kinds of
questions about semiconductor diodes posed - and answered -
in this article in a 1961 issue of Popular Electronics.
Author Jim Kyle runs through a short history of he diode and
then delves with more detail into physical construction, I-V
curves, power handling, junctions capacitance, resistance, etc.
An interesting point mentioned is that while a semiconductor
diode will conduct some finite amount of current when biased
in the reverse direction (sometimes a desired characteristic),
a vacuum tube diode will not conduct at all when reverse biased
- thereby making the tube a more perfect rectifier.

A seven story high intercontinental missile roars skyward
on a column of fire. Within the silvery giant, hundreds of tiny
semiconductor diodes control its every movement.

A television camera is focused on a man. Millions of viewers
are watching. Between the man and the millions of viewers are
dozens of semiconductor diodes - without them television could
not function.

Older than radio itself, and once thought obsolete, semiconductor
diodes today are the workhorses of the electronics industry.
They form the heart of nearly all digital computers - the giant
electronic brains that can predict an election outcome or control
a manufacturing plant. They make radar possible. They detect
radio signals, and, on occasion, generate those same signals.

What are these devices? How do they work? What are their
characteristics? How are they used?

In essence, the answers are simple. First of all, a semiconductor
diode is a one-way street for electric currents. It will allow
the current to flow freely in one direction, but will block
it almost completely in the other. Because of this characteristic,
the semiconductor diode can perform a wide variety of jobs and
is one of our most basic electronic servants.

How The Diode Works

To understand how a semiconductor diode works, let's go back
a bit and examine electricity itself. An electric current is
simply another name for a flow of electrons - the basic electrical
charge found in all elements. Electricity flows when electrons
move from one atom of a substance to the next.

In some materials - copper, silver, aluminum, and many other
metals - the electrons can move easily. These substances are
called conductors.

In other materials - glass, porcelain, hard rubber, and many
plastics - the electrons can move only with great difficulty.
In fact, only a very few electrons can move at all in these
substances, even under great electrical pressure; and so flow
of electric current through them is blocked. We call these substances
insulators.

Between conductors and insulators are many materials which
are neither good conductors nor acceptable insulators. The electrons
of their atoms are free to move, but are not so free as in a
conductor. These substances are known as semiconductors.

Types of Semiconductors. Although many semiconductors
exist (most materials fall into this classification), only a
few are used in electronics. Those most widely used are germanium,
silicon, selenium, and copper oxide. In past years, galena (a
form of lead oxide) was also used.

Selenium rectifiers, in use for over 25 years,
are giving way to smaller silicon diodes such as Sarkes-Tarzian's
1N1083.

These particular semiconductors have a strange property.
Under certain special conditions, electrons can flow out of
them easier than in. Under other conditions, the situation is
reversed: electrons come in freely, but have difficulty getting
out.

Since this strange property makes itself evident only when
electrons enter or leave the semiconductor material, it is useful
only when the semiconductor is in contact with a conductor.
This contact may be made in two ways: by point contact, in which
the semiconductor and the conductor make contact at only a single
point; and by surface contact, in which they meet over a broad
area. Each way has its advantages.

An early example of point-contact use is the old-fashioned crystal
set. Invented about 1906 by two experimenters named H. H. Dunwoody
and G. W. Pickard, this was the mainstay of radio for nearly
20 years. It consisted of a small piece of galena crystal and
a spring-wire "cat-whisker." The user moved the cat-whisker
over the surface of the crystal until a sensitive spot was located.

An example of surface-contact application is the copper-oxide
stack, widely used in both test equipment and in telephone engineering.
Developed about 1925, this device consists of alternate discs
of lead and copper oxide, stacked face-to-face and held together
by an insulated bolt through the center. It requires no adjustment.
However, technical limitations restrict its use.

Another example of surface-contact diodes is the modern grown-junction
unit, such as the 1N34 so widely used by experimenters.

Electron Flow. At this point, let's narrow
the field down to a typical point-contact unit such as the crystal
set and take a look at what happens when this semiconductor
diode is connected to a battery and a meter. See Fig. 1.

Fig. 1. - Diode current measurement

When the battery is connected, its voltage forces electrons
of the interconnecting wire into the semiconductor, across the
contact point, into the conductor, through the meter, and back
through the other interconnecting wire into the battery.

You can see that with the battery connected in one direction,
electrons are forced out of the semiconductor at the contact
point. If the battery's polarity is reversed, electrons will
be forced into the semiconductor.

Let's assume that this particular diode is made from a semiconductor
that is stingy with electrons; that is, it accepts electrons
readily, but doesn't let go of them so easily.

When the battery is connected in the first direction, forcing
electrons out of the semiconductor at the contact point, the
semiconductor material exhibits great resistance. Only a few
electrons are released to travel on through the meter and back
to the battery, and so only a small current flows.

However, when the battery is reversed, we're pushing electrons
into our greedy semiconductor, and it readily accepts all we
can offer. Many electrons move through the meter, or, in other
words, a large current flows.

Only the action at the contact point is important; the other
electrical connection to the semiconductor material covers a
much larger area and, since resistance is proportional to area,
has much lower resistance. However, it does contribute to the
diode's forward resistance, which we'll talk about more a bit
later.

If a different semiconductor - one that is generous instead
of miserly - is used, the situation will be exactly opposite
to that described above. However, the diode would still be a
one-way street. The only difference is that it would be one-way
in the other direction.

This one-way-street action is similar in effect to the action
of a diode vacuum tube, such as the familiar type 5U4-G. In
the vacuum tube, heat generated in the filament causes electrons
to literally boil off its surface. When the plate of the tube
is made positive, the electrons flow to it. However, since like
charges repel each other, the electrons will not go to the plate
when it is negative.

Pros and Cons. In both the semiconductor
diode and its vacuum-tube cousins, current flows readily in
only one direction. This property makes them useful in changing
a.c. to d.c., and they are widely used in electronic power supplies
for this reason.

A great advantage of the semiconductor diode over its vacuum-tube
cousins is that the semiconductor version does not require heat
to move its electrons. This eliminates the hot and power-wasting
filament.

Another advantage is the smaller size possible with semiconductors.
Typical semiconductor diodes are no bigger around than a pencil,
and less than an inch long - compared to the 3/4" diameter and
1 1/4" length of the smallest standard vacuum diodes.

Another point of difference between the semiconductor diode
and its vacuum-tube cousins - but it's not usually considered
an advantage - is the matter of reverse current.

In the semiconductor diode, current flows more easily in
one direction than in the other. However, in the vacuum-tube
version, current can flow only in one direction. While the semiconductor
diode is like a one-way street for electrons, the vacuum diode
is more like a subway turnstile. You can go the wrong way on
a one-way street; you can't go the wrong way through a turnstile.

While this might look like a big disadvantage for the semiconductor
diode, it usually isn't harmful in practice. Present-day diodes
may pass a million times as much current in one direction as
in the other; the small number of electrons which get though
the wrong way have little or no effect on diode operation.

Since the point-contact diode is the oldest type, the standard
schematic symbol for a semiconductor diode is based on it. See
Fig. 2.

Regardless of whether the semiconductor is stingy or generous
with electrons, the arrow of the symbol points against the flow
of traffic in our one-way street. This confusing situation came
about in earlier years, before scientists had learned as much
about the diode as they know today. The original direction for
the arrow was chosen arbitrarily, and the symbol had been in
use for some time before they discovered the arrow was pointing
the wrong way!

Characteristics

The main property of a semiconductor diode is that it will
pass current easily in one direction, and will allow only a
small amount of current to flow the other way. The easy-current
direction is usually called forward, while the other direction,
quite naturally, is called reverse.

Current. One of the basic characteristics on which these
diodes are rated is the amount of current which the unit will
let through in each direction. The ratings are listed in terms
of forward current and reverse current. Forward current, i.e.,
current going in the easy direction, is always the larger of
the two. Frequently forward current is measured in hundreds
of milliamperes while reverse current is given in microamperes.

Another way of looking at these diodes is to examine their
resistance. Since resistance (in ohms) is equal to the applied
voltage divided by the current (in amperes) which flows through
the circuit, you can see that resistance in the forward direction
is much lower than resistance in the reverse direction. The
more common way of putting this is to say that forward resistance
of a semiconductor diode is low while reverse or back resistance
is high.

Resistance. However, semiconductor diodes
have an unusual resistance characteristic. Their resistance
varies in accordance with the voltage you apply to them. At
low voltages, forward resistance is high; at higher voltages,
it drops. Reverse resistance, on the other hand, is extremely
high at low voltages, but drops to zero or even exhibits negative
characteristics at some critical point as voltage increases.

The critical point at which reverse resistance tends to disappear
is called the diode's peak inverse voltage (usually abbreviated
PIV) and is a key characteristic of power rectifiers.

Engineers call a resistance characteristic of a semiconductor
diode nonlinear, because the plotted line on a graph comparing
voltage against current appears as a curve instead of a straight
line. The nonlinear resistance of the semiconductor diode makes
it useful as a detector, as a mixer, and as a modulator; however,
the nonlinear resistance also makes it difficult to specify
any other diode characteristic. For instance, a vacuum diode
can be rated for 300 milliamperes of current, and this will
be true at any voltage. Before a semiconductor diode can be
rated, however, you must specify the voltage.

The same is true of the all-important reverse resistance
rating. The same diode may have a reverse resistance of one
megohm, less than an ohm, or even negative 100 ohms, depending
entirely upon the voltage at which the reading is taken.

Voltage. All diode characteristics, therefore,
are given in terms of current at some specified voltage. Different
manufacturers use different voltages, and to complicate things
still more, some firms rate different diodes at different voltages.
This makes comparison of two diodes, on the basis of rated characteristics,
almost impossible unless both are rated under the same conditions.

Fig. 3. - Semiconductor diode I-V curve.

Manufacturers of diodes, however, furnish another item which
can help you avoid this problem - the characteristic curve of
the diode. See Fig. 3.

The vertical scale in Fig. 3 shows current; the horizontal
scale, voltage. Note that forward voltages and currents are
expressed in larger units than are reverse values; this is customary
in the preparation of diode characteristic curves.

With a set of characteristic curves, you can determine the
characteristics of a diode at any operating point. Simply look
up the current value for the voltage you intend to use, and
determine the resistance by using Ohm's law. To compare two
different diodes, compare the shape of the curves.

Bias. A term worth mentioning at this point,
since you'll hear it frequently in dealing with semiconductor
diodes, is bias. Bias consists of a voltage applied to a diode
to make it operate at the point desired by the designer. If
the voltage applied causes forward current to flow, it's called
forward bias. If it's applied in the reverse direction, the
term is reverse bias. A diode to which such a voltage is applied
is said to be biased.

In addition to the major characteristics which we've examined
so far - forward current, reverse current, forward resistance,
reverse resistance, and peak inverse voltage - semiconductor
diodes have two more important characteristics. They are the
thermal character­istic and diode capacitance.

Temperature. "Thermal characteristic" simply
is a fancy way of saying "how heat affects the diode." We stated
earlier that in the vacuum-tube diode electrons were boiled
off the filament by heat. Actually, heat increases the movement
of all electrons, something like popcorn on a hot stove. At
high temperatures, the electrons move more freely.

Up to a certain point, heat has little effect on a semiconductor
diode. Although reverse current increases slightly, forward
current increases in the same ratio. At the critical temperature,
however, the crystal structure breaks down and current flows
just as freely in either direction. Some diodes recover when
they are cooled, while others are ruined for good.

The manufacturer usually rates his product to be used within
a certain temperature range, and this range is generally far
greater than the temperatures at which you are likely to use
it (a typical operating range is from 40 degrees below zero
to 300 degrees above). However, if excessive current is sent
through the diode in either direction, it may heat - internally
- to a point far above the critical breakdown temperature. This
is the most frequent cause of diode failure.

Capacitance. The last major characteristic
is diode capacitance. A capacitor, by definition, is made up
of two conductors separated by a dielectric. Thus, the semiconductor
material itself can be the dielectric of a capacitor whose plates
are the conductors on either side.

Actually, the physicists tell us, most semiconductor diodes
show a greater capacitance than we would expect, due to something
called the barrier effect. This is a function of the applied
voltage. As a result, the capacitance of a semiconductor diode
changes with the voltage in a manner similar to the diode's
resistance.

This capacitance has little effect on forward resistance
or forward current, since the diode is conducting and the capacitance
is shorted out. However, the diode's capacitance can be important
when the diode is not conducting, since the capacitance will
allow very-high-frequency alternating currents to pass.

Most semiconductor diodes have a capacitance in the neighborhood
of 3 to 5 μμf. Those diodes built especially for use at
radar frequencies have even less capacitance.

How Diodes are Used

Of what practical use is the semiconductor diode's one-way-street
property? One of the more obvious applications is in changing
alternating current into direct current, such as in a receiver's
power supply. The diode is simply connected in series with the
a.c. coming from the transformer. Those half-cycles which constitute
forward voltage go through the diode into the filter circuit,
while half-cycles of inverse voltage are blocked.

Fig. 4. - Half-wave diode rectifier.

Half-Wave Rectifier. The circuit in Fig.
4, called a half-wave rectifier, is the simplest possible, but
it is hardly the most efficient. Half of the a.c. power is not
used. However, by connecting three additional diodes into a
"bridge" circuit, the half-cycles of opposite-polarity a.c.
can be steered in the proper directions so that both halves
of each cycle are used and yet the power supplied to the filter
is direct current. Refer to Fig. 5.

When the voltage at point A in Fig. 5 is positive, the voltage
at point B will be negative, since the supply voltage is alternating.
The electrons flow from point B through diode D2 to the filter
and load circuit, and are blocked at D3 and D4 since their reverse
resistance is high. From the load and filter, the electrons
return through diode D1 to point A.

On the other half-cycle, electrons flow from point A through
diode D4 to the filter - being blocked at diodes D1 and D2 by
the reverse resistance - then return to point B through D3.

Fig. 5. - Full-wave diode rectifier.

Other rectifier circuits using more than one diode are popular.
They include voltage multipliers which make it possible to obtain
as much as 1000 volts of direct current from a 117-volt power
line without using transformers, dual-voltage circuits which
provide two different direct voltages from one transformer,
and "bias-supply" circuits which can be made to fit into less
space than a conventional vacuum-tube rectifier alone.

The semiconductor diode most widely used for power supply
rectifiers is the selenium stack. However, silicon junction
diodes capable of handling four to five times the current of
the selenium stack in one-tenth the space are rapidly becoming
popular.

Diode Detector. The semiconductor diode
also finds wide use in radio receivers, television sets, radar,
and test equipment, as a detector of r.f. power.

The circuit of the diode detector is identical to that of
the half-wave rectifier - it contains a source of power, the
diode, and a load, all connected in series. However, the operation
is slightly different.

Small semiconductor diodes have axial leads,
eliminating the need for sockets.

During each cycle of r.f. energy, the diode allows current
to pass in the forward direction but blocks the flow of reverse
current. The current flowing in the forward direction produces
a voltage drop in the load resistor which is paralleled with
a low-value capacitor. If the strength of the r.f. power is
changing, the d.c. voltage across the load resistor will change
at the same rate. And if this change occurs at an audio frequency,
the voltage across the load resistor will vary at the same a.f.
rate.

The average strength of the d.c. voltage across the detector
load resistor is proportional to the average strength of the
r.f. voltage applied to the circuit. In radio receivers, this
effect is used to provide automatic volume control, while in
test equipment it is used to measure r.f. with a conventional
d.c. voltmeter.

Vacuum-tube diodes, which operate in similar fashion, can
be used for these purposes at moderately high frequencies. However,
at the extremely high frequencies used in radar, they fail to
function properly. Here, the semiconductor diode's very low
capacitance makes it the only usable detector.

Fig. 6. - Diode AND gate.

Computer Circuits. A few paragraphs earlier,
we met the bridge rectifier circuit, and saw how the semiconductor
diode was capable of steering an incoming signal into one of
several directions. This property is widely used in computer
circuitry, where the proper combination of diodes can actually
make logical decisions.

A basic circuit of this type is shown in Fig. 6. This circuit
is exceptionally choosy - it will produce an output signal only
if you give it signals at both of its inputs. If you give it
a signal at only one of the input terminals, it produces nothing.
This is called a logical "and" circuit, since it must have both
signal A and signal B to provide an output. Another way of putting
it is to say that the circuit must decide whether both inputs
are present before deciding to produce an output.

With no input signals applied, both diodes are biased in
the forward direction by the positive voltage through R2; R1's
value is very much less than that of R2, so the output is nearly
zero. With a positive input signal applied to either A or B
but not to both, the diode without an input signal still shorts
the voltage from R2 through R1 to ground and no output is developed.
However, with positive input signals applied to both A and B
at the same time, both diodes are biased in the reverse direction.
Current through R2 meets the high reverse resistance of the
diodes, and in consequence is shunted through the output circuit.

Typical values for R1 and R2 are 10 ohms and 10,000 ohms,
respectively. The voltage source is usually about 12 volts.

Similar circuits are used to develop outputs if a signal
is applied to either input; to develop output if a signal is
applied to either input but not to both; and to develop output
at all times except when a signal is applied to both inputs.

Circuits such as these form the basis of many of the giant
computers. Each circuit is simple enough, but a typical computer
may contain literally thousands of them. The semiconductor diode
makes this possible; if you were to try to use vacuum diodes
in its place, you would find that filament power requirements
alone would mount to hundreds of kilowatts!

Automatic Noise Limiter. Another use of
the semiconductor diode's "gating" ability is in the automatic
noise limiter found in many ham-type radio receivers. The purpose
of the noise limiter is to steer the desired audio signals to
the loudspeaker, and to gate any noise bursts caused by passing
cars or by static crashes to ground.

Fig. 7. - Diode noise limiter.

While dozens of noise-limiter circuits exist, the circuit
shown in Fig. 7 is one of the simplest and is unusually effective
on many types of noise.

With no noise, the limiter diode is forward-biased by the
d.c. voltage developed across the detector load resistor, and
it conducts until the capacitor charges to the value of this
voltage. At this point, the bias on the limiter diode drops
to zero.

Remember that when we were discussing the nonlinear resistance
of the diode, we found it had high forward resistance at low
voltages, and low resistance at higher voltages? With no noise,
and consequently no bias, the diode's resistance is high and
the capacitor is effectively out of the audio circuit.

However, when a noise pulse - whose voltage is much higher
than the average signal - comes along, the picture changes.
The diode is once more biased to a low-resistance point, and
gates the noise pulse through the capacitor to ground. As soon
as the pulse is gone, diode resistance returns to its normal
high value.

Mixers. Semiconductor diodes are also widely
used as mixers in extremely high frequency superhet receivers,
such as radar and microwave-relay sets. In this application,
they outperform any available tube. In fact, much of the progress
that separates today's semiconductor diodes from the ancient
crystal set can be traced to World War II development of the
diode for use in radar sets as the mixer element.

A complete explanation of this form of diode operation requires
pages of mathematical equations; in simplified form, this is
how it works:

The diode is connected to the antenna of the set, and is
also connected to a local oscillator whose frequency is separated
from that of the incoming signal by some small, desired amount.
Signals coming from the antenna mix, in the diode, with those
from the local oscillator.

The diode's output, you can see, will consist of pulses of
direct current occurring on each half-cycle of the antenna signal,
and other pulses every half-cycle of the local-oscillator signal.
In addition, however, two new signals are created. Their frequencies
are equal to the sum and the difference of the antenna and local-oscillator
signals, and their strength is proportional to the product of
the two input signals.

Since only the incoming antenna signal is changing in strength,
the difference signal will be a replica of the antenna signal
but at a lower frequency. Thus, the tricky microwave signal
is converted to a lower frequency signal which can be handled
by more conventional means.

Semiconductor diodes excel as microwave mixers because of
their extremely low capacitance. Other types of mixer circuits
fail to operate at frequencies higher than about 900 megacycles,
but semiconductor diode mixers continue to operate up to 30,000
mc., and some new types promise to work at even higher frequencies.

Special Uses. Many special-use circuits
have been developed around semiconductor diodes. Telephone engineers
use diodes as modulators, making use of the nonlinear resistance.
Under certain conditions, the resistance of a diode can become
negative - and it can then be used as an oscillator. Under other
conditions, diode capacitance can be varied at an extremely
rapid rate - and this leads to the "parametric amplifier" which
makes possible communication by moon-bounce and radar contact
with distant planets.

Packaging of diodes in tube shells permits
replacement of 5U4, 5W4, 5Y3, 6X4 and other rectifiers. Cartridge-cased
diodes are used in the newer TV sets.

"Special" Diodes. The many uses we've listed
so far for the semiconductor diode barely begin to show the
variety of jobs to which this electronic workhorse is hitched
daily. In addition to conventional diodes such as we have discussed,
there are dozens of "special" diodes in which one characteristic
or another is stressed, and more new types are being developed
every month.

Among these "special" diodes are the tunnel diode, which
operates at speeds near that of light; the Zener diode, which
can regulate voltages in the same manner as a VR tube; and the
voltage-variable capacitor - which is really a diode at heart.

Yes, the semiconductor diode has traveled a long way since
its original discovery in 1874, 13 years before Dr. Heinrich
Hertz discovered radio itself. From the primitive crystal set
and crude copper-oxide stacks, through the sealed-unit microwave
mixers of World War II and into the era of the junction diode
(announced in 1948), it has been one of the most basic, most
useful, and least understood of our electronic servants. Over­shadowed
in the early 1920's by its bigger and hotter rival, the vacuum
tube, the semiconductor diode is only now regaining its place-as
electrons' one-way street.

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